This invention relates to magnetic resonance imaging apparatus.
Such apparatus comprises a magnet for producing a main magnetic field to align magnetic resonant (MR) active nuclei such as hydrogen in tissue in a patient or object, and r.f. excitation means to excite these nuclei to resonance. The resulting relaxation signals generated by the nuclei are picked up by a receive coil, and provide information on the distribution of the nuclei and hence information about the tissue itself. The signal is spatially encoded by means for generating magnetic field gradients.
Typically, only a selected region of the patient is excited, but noise is generated from all parts of the patient, whether excited or not. In the interests of improving the signal-to-noise ratio, it has been proposed (U.S. Pat. No. 4,851,777, U.S. Pat. No. 4,682,112) to provide an array of receive coils, the relative amplitude and phases of the signals from which being such that certain regions are nulled out i.e. signal is only collected from the region of interest. It has also been proposed in the same publications to use such an array of coils to excite only the region of interest. This would then avoid unnecessary heat input to the remainder of the patient.
In any magnetic resonance imaging apparatus, the region selected for imaging is within the region of good field produced by the magnet system. The requirements for the region of good field is that the field of the main magnet must be homogeneous to a very high degree, and the magnetic field gradients imposed on the main field must be uniform to a very high degree.
The steepness of the magnetic field gradient in the axial direction may be influenced by the design of the imaging apparatus. Thus, in the interests of patient comfort, the axial length of magnets is being reduced, to avoid the need for the patient to be accommodated in a deep bore which could cause problems of claustrophobia. The axial magnetic field gradient, which usually defines the "slice" of the patient being imaged must necessarily then be steeper than hitherto. The gradient falls off outside the region of good field, but because the gradient is steeper, the fall off now takes place closer to the region of good field than hitherto.
The particular slice selected in the region of good field is determined by the frequency of the r.f. excitation pulse which excites nuclei precessing at a certain frequency. The frequency at which the nuclei precess in turn is dependent on the magnetic field strength i.e. the point along the gradient in the axial direction. Unfortunately where the magnetic field strength falls off, it will pass through the same value as that which defines the selected slice. This region will also therefore be excited, and will impair the desired image picked up by the receive coil. This so-called aliasing is a known problem, but the shorter magnet makes it worse.